1. Field of the Invention
The present invention relates to an OFDM receiver apparatus for receiving and demodulating an OFDM signal, and specifically to a demodulator circuit for demodulating a DBPSK signal in an OFDM receiver apparatus for digital terrestrial broadcasting and a circuit for detecting frame synchronization.
2. Description of the Related Art
In recent years, OFDM (Orthogonal Frequency Division Multiplexing) has been proposed as a method for transmitting digital signals. In OFDM, data is transmitted by using plural carriers, which are orthogonal to each other in a frequency domain. For that reason, an OFDM transmitter apparatus modulates transmitting signal using IFFT (Inverse Fast Fourier Transformation), and an OFDM receiver apparatus demodulates the received signal using FFT (Fast Fourier Transformation). Because frequency use efficiency is high, application of OFDM to the digital terrestrial broadcasting has been under consideration at large. Note that OFDM is adopted in ISDB-T (Integrated Services Digital Broadcasting-Terrestrial), which is a standard of the digital terrestrial broadcasting in Japan. OFDM is also adopted in DVB-T or DVB-H in Europe.
In the digital terrestrial broadcasting, control information is transmitted together with data. The data is transmitted by data carriers, and the control information is transmitted by TMCC (Transmission and Multiplexing Configuration Control) carriers. Here, the control information contains information for detecting frame synchronization, information for demodulating data signals and others. Therefore, accurate determination of the TMCC data transmitted by the TMCC carriers is important.
FIG. 1 is a diagram explaining a conventional data decision method. It should be noted that the TMCC signal is a DBPSK (Differential Binary Phase Shift Keying) signal. In DBPSK, a 1-bit digital signal is transmitted by using a phase difference (“zero” or “π”) between two successive symbols. The OFDM signal of the digital terrestrial broadcasting contains plural TMCC signals for transmitting the same information in parallel in order to raise reliability.
In FIG. 1, an FFT circuit 101 converts an OFDM signal into a frequency domain signal. As a result, a data signal, a TMCC signal and others can be obtained. A phase difference calculation circuit 102 sequentially calculates phase differences between symbols of the TMCC signal obtained by the FFT circuit 101. A BPSK demodulator 103 generates 1-bit digital data for every symbols based on the phase difference information obtained by the phase difference calculation circuit 102. Here, the phase difference calculation circuit 102 and the BPSK demodulator 103 perform the above processing on each of the TMCC signals. Consequently, plural 1-bit data can be obtained for every 1-symbol time. A majority decision circuit 104 performs a majority decision procedure on the plural 1-bit data, and outputs likelihood data (data in the majority). As a result, the TMCC data is regenerated. As an example shown in FIG. 2, in a system where 12 TMCC signals are transmitted in parallel, “0” is detected from eight TMCC signals and “1” is detected from four TMCC signals. In such a case, the transmission data is determined as “0”.
As described above, the TMCC signal that requires high reliability is demodulated by using the majority decision. Note that a technique to demodulate the TMCC signal is described in Patent Document 1 (Japanese Patent Application Publication No. 2002-247003), for example. In the demodulating circuit described in Patent Document 1, the majority decision is made by only using TMCC signals with the reception level larger than a threshold.
A receiver apparatus of the digital terrestrial broadcasting establishes frame synchronization by utilizing synchronization data contained in the TMCC data. The data is generated by executing demodulating processing with synchronization timing as a reference.
FIG. 3 is a diagram explaining a conventional frame synchronization detection method. Note that the synchronization data in ISDB-T is “w0=0011010111101110” or “w1=1100101000010001”.
In FIG. 3, a known data register 111 stores synchronization data w0 and w1. The TMCC data obtained as above is sequentially input to a shift register 112. In other words, the shift register 112 holds the latest 16-bit TMCC data in sequence. A comparator 113 compares the synchronization data stored in the known data register 111 with the 16-bit data held in the shift register 112. If the number of bits matching each other is larger than the predetermined threshold (e.g. 14), a synchronization signal indicating the establishment of frame synchronization is output. Note that a known method includes that when a condition in which the number of bits matching each other is larger than the threshold is detected successively for a prescribed times, establishment of the frame synchronization is determined.
As related technologies, Patent Document 2 (Japanese Patent Application Publication No. H07-250120) describes a method for establishing the frame synchronization. Patent Documents 3 (Japanese Patent Application Publication No. 2000-115122) and 4 (Japanese Patent Application Publication No. 2000-299676) describe a method for establishing the symbol synchronization in an OFDM receiver apparatus.
In a case of receiving the digital terrestrial broadcasting with a mobile terminal such as a mobile phone, there is a frequent occurrence of fading phenomena in which the reception level of the radio waves fluctuates. Because reception power is reduced under a strongly fading environment, sometimes signals cannot be demodulated temporarily. At that time, if the TMCC information regeneration fails, the receiver apparatus can not recognize a demodulation method, the transmission data would not be regenerated at all.
Similarly under a strongly fading environment, the synchronization data is not accurately regenerated, and there is a possibility that the frame synchronization cannot be established. In this case, again, the transmission data cannot be regenerated at all. If the threshold for the synchronization decision is lowered (i.e. if some errors are accepted), the synchronization would be more easily established. However, the lower threshold increases the likelihood of erroneous synchronization (i.e. synchronization signal is output at a wrong timing).
The conventional OFDM receiver apparatus, as described above, sometimes cannot perform demodulation processing or frame synchronization establishment under a strongly fading environment.